btrfs: sync some cleanups from progs into uapi/btrfs.h

When syncing this code into btrfs-progs Dave noticed there's some things
we were losing in the sync that are needed.  This syncs those changes
into the kernel, which include a few comments that weren't in the
kernel, some whitespace changes, an attribute, and the cplusplus bit.
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

include/uapi/linux/btrfs.h

@@ -19,8 +19,14 @@
 
 #ifndef _UAPI_LINUX_BTRFS_H
 #define _UAPI_LINUX_BTRFS_H
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 #include <linux/types.h>
 #include <linux/ioctl.h>
+#include <linux/fs.h>
 
 #define BTRFS_IOCTL_MAGIC 0x94
 #define BTRFS_VOL_NAME_MAX 255
@@ -333,6 +339,12 @@ struct btrfs_ioctl_feature_flags {
  */
 struct btrfs_balance_args {
 	__u64 profiles;
+
+	/*
+	 * usage filter
+	 * BTRFS_BALANCE_ARGS_USAGE with a single value means '0..N'
+	 * BTRFS_BALANCE_ARGS_USAGE_RANGE - range syntax, min..max
+	 */
 	union {
 		__u64 usage;
 		struct {
@@ -549,7 +561,7 @@ struct btrfs_ioctl_search_header {
 	__u64 offset;
 	__u32 type;
 	__u32 len;
-};
+} __attribute__ ((__may_alias__));
 
 #define BTRFS_SEARCH_ARGS_BUFSIZE (4096 - sizeof(struct btrfs_ioctl_search_key))
 /*
@@ -562,6 +574,10 @@ struct btrfs_ioctl_search_args {
 	char buf[BTRFS_SEARCH_ARGS_BUFSIZE];
 };
 
+/*
+ * Extended version of TREE_SEARCH ioctl that can return more than 4k of bytes.
+ * The allocated size of the buffer is set in buf_size.
+ */
 struct btrfs_ioctl_search_args_v2 {
 	struct btrfs_ioctl_search_key key; /* in/out - search parameters */
 	__u64 buf_size;		   /* in - size of buffer
@@ -570,10 +586,11 @@ struct btrfs_ioctl_search_args_v2 {
 	__u64 buf[];                       /* out - found items */
 };
 
+/* With a @src_length of zero, the range from @src_offset->EOF is cloned! */
 struct btrfs_ioctl_clone_range_args {
-  __s64 src_fd;
-  __u64 src_offset, src_length;
-  __u64 dest_offset;
+	__s64 src_fd;
+	__u64 src_offset, src_length;
+	__u64 dest_offset;
 };
 
 /*
@@ -677,8 +694,11 @@ struct btrfs_ioctl_logical_ino_args {
 	/* struct btrfs_data_container	*inodes;	out   */
 	__u64				inodes;
 };
-/* Return every ref to the extent, not just those containing logical block.
- * Requires logical == extent bytenr. */
+
+/*
+ * Return every ref to the extent, not just those containing logical block.
+ * Requires logical == extent bytenr.
+ */
 #define BTRFS_LOGICAL_INO_ARGS_IGNORE_OFFSET	(1ULL << 0)
 
 enum btrfs_dev_stat_values {
@@ -1144,4 +1164,8 @@ enum btrfs_err_code {
 #define BTRFS_IOC_ENCODED_WRITE _IOW(BTRFS_IOCTL_MAGIC, 64, \
 				     struct btrfs_ioctl_encoded_io_args)
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif /* _UAPI_LINUX_BTRFS_H */